1. Field of the Invention
The present invention relates to a method of manufacturing a surface mounted device (SMD)-type piezoelectric vibrator in which a piezoelectric vibrating reed is sealed in a cavity formed between two bonded substrates, and an oscillator, an electronic device, and a radio-controlled timepiece each having the piezoelectric vibrator.
2. Description of the Related Art
In recent years, a piezoelectric vibrator utilizing crystal or the like has been used in mobile phones and mobile information terminals as the time source, the timing source of a control signal, a reference signal source, and the like. Although there are various piezoelectric vibrators of this type, a surface mounted device-type piezoelectric vibrator is known as one example thereof. As the piezoelectric vibrator of this type, generally, a three-layered piezoelectric vibrator in which a piezoelectric substrate having a piezoelectric vibrating reed formed thereon is bonded so as to be interposed from above and below by the base substrate and a lid substrate is known. In this case, the piezoelectric vibrator is accommodated in a cavity (sealed space) that is formed between the base substrate and the lid substrate. Moreover, in recent years, instead of the three-layered piezoelectric vibrator described above, a two-layered piezoelectric vibrator has also been developed.
The piezoelectric vibrator of this type has a two-layer structure in which a base substrate and a lid substrate are directly bonded, and a piezoelectric vibrating reed is accommodated in a cavity formed between the two substrates. The two-layered piezoelectric vibrator is ideally used as it is superior in achieving a thin profile compared with the three-layered structure. As an example of such a two-layered piezoelectric vibrator, a piezoelectric vibrator in which a piezoelectric vibrating reed is electrically connected to external electrodes formed on a base substrate using a conductive member which is formed so as to penetrate through the base substrate is known (for example, see Patent Citations 1 and 2).
This piezoelectric vibrator 200 includes a base substrate 201 and a lid substrate 202 which are anodically bonded to each other by a bonding film 207 and a piezoelectric vibrating reed 203 which is sealed in a cavity C formed between the two substrates 201 and 202, as shown in FIGS. 28 and 29. The piezoelectric vibrating reed 203 is a tuning-fork type vibrating reed, for example, and is mounted on the upper surface of the base substrate 201 in the cavity C by a conductive adhesive E.
The base substrate 201 and the lid substrate 202 are insulating substrates, for example, made of ceramics, glass, and the like. Among the two substrates 201 and 202, through holes 204 are formed on the base substrate 201 so as to penetrate through the base substrate 201. Moreover, a conductive member 205 is embedded in the through holes 204 so as to block the through holes 204. The conductive member 205 is electrically connected to external electrodes 206 which are formed on the lower surface of the base substrate 201 and is electrically connected to the piezoelectric vibrating reed 203 mounted in the cavity C.    Patent Citation 1: JP-A-2001-267190    Patent Citation 2: JP-A-2007-328941
In the two-layered type piezoelectric vibrator described above, the conductive member 205 performs two major roles of blocking the through holes 204 to maintain the airtightness in the cavity C and electrically connecting the piezoelectric vibrating reed 203 and the external electrode 206 to each other. In particular, if the contact between the conductive member 205 and the through holes 204 is not sufficient, there is a possibility that the airtightness in the cavity C is impaired. Moreover, if the contact between the conductive member 205 and the conductive adhesive E or the external electrode 206 is not sufficient, the piezoelectric vibrating reed 203 will not operate properly. Therefore, in order to eliminate such a problem, it is necessary to form the conductive member 205 in a state where the conductive member 205 is tightly and closely adhered to the inner surfaces of the through holes 204 to completely block the through holes 204, and no depression or the like appears on the surface.
Patent Citations 1 and 2 describe that the conductive member 205 is formed using a conductive paste (an Ag paste, an Au—Sn paste, or the like). However, there is no description as to a specific manufacturing method such as how to form the conductive member.
In general, when a conductive paste is used, it is necessary to perform baking to cure the conductive paste. That is, it is necessary to perform baking to cure the conductive paste after it is filled in the through holes 204. However, when baking is performed, since organic materials included in the conductive paste are removed through evaporation, the volume of the conductive paste after baking generally decreases compared to the volume of the conductive paste before baking (for example, the volume decreases approximately 20% when an Ag paste is used as the conductive paste). Therefore, even when the conductive member 205 is formed using the conductive paste, there is a possibility that depressions appear on the surface, or in severe cases, the centers of the penetration holes are opened. As a result, there is a possibility that the airtightness in the cavity C is impaired, or the electrical connection between the piezoelectric vibrating reed 203 and the external electrode 206 is impaired.
In order to solve the problems described above, a technique of filling a glass paste in the through holes 204 in a state where a conductive rivet member is inserted into the through holes 204 and integrating the glass paste with the through holes 204 by performing baking is proposed.
However, there is a problem in that if the contact surfaces of the rivet member and the base substrate 201 are not sufficiently in contact with each other when filling the glass paste, the glass paste leaks. Moreover, there is also a problem in that a load is applied to the contact surfaces of the rivet member and the base substrate 201 when filling the glass paste in the through holes 204, cracks are formed in the base substrate 201. Therefore, there is a problem in that the yield decreases when manufacturing the piezoelectric vibrator.